An optical transmission system may use low frequency tones to identify the different wavelengths of optical signal that it is transmitting over an optical path, e.g., an optical fiber. This is done, for example, to control the level of power that an optical signal amplifier outputs to an optical fiber on a per channel basis, and thus prevent overpowering the optical channel. Such overpowering typically causes the system to degrade as a result of so-called self-phase modulation which is induced by a non-linear fiber response in the optical channel. This problem is especially acute in an optical transmission system spanning an appreciable distance and employing a number of optical amplifiers to regenerate the optical signals at various points along the transmission system. For example, if a system has a single optical channel, then an optical amplifier, in response to receipt of an optical signal via that channel, may inject a relatively strong optical signal, e.g., a 16 dBm signal, into the associated optical fiber. It is likely then that the strong optical signal will cause the fiber to respond nonlinearly and thus severely degrade signals that are being transported over that channel. Thus, optical transmission systems use tones to identify which channels are active and to control the amplification of an optical signal.
We have recognized, however, that when the transmitted signals are received at a receiving node and demultiplexed to "drop" one or more channels for re-transmission to another optical node, each demultiplexed signal (wavelength) carries all of the transmitted identifications signals. Disadvantageously, circuits in the path to the other node as well as the other node may incorrectly conclude that the received signal includes more channels that it actually does as a result of the received signal carrying all of the originally transmitted identifications signals.